The length of a semi-trailer for transporting containers closely relates to the length of the containers. For example, a domestic container of inland America is 53 feet in length, so that a semi-trailer for transporting the container should be longer than 53 feet in length. There are two methods for shipping these extremely long semi-trailers. The first method is to use bulk cargo ships and the second method is to use container vessels. By using bulk cargo ships, the semi-trailers can be transported integrally. However, the first method occupies substantial space and the freight cost is high. Additionally, the shipping frequency of bulk cargo ships is too low to meet delivery requirements. With the second method, the shipping frequency of the container vessels is high, and the freight cost is low, but there are limitations on cargo of extreme length. Since the length of the ISO container is 20 or 40 feet, in order to transport the semi-trailer by using the ISO containers, the semi-trailer should be segmented into two or more parts, and assembled after arriving at its destination to form a complete semi-trailer. Usually, the segmented semi-trailers are assembled by using joints. FIG. 1 illustrates the structure of a two-segment semi-trailer for transporting 53′ containers. Because the main beam of the semi-trailer is too long to be transported as a whole, the main beam 1 is segmented into a front beam 1a and a rear beam 1b. After reaching the destination, the front beam 1a and the rear beam 1b are assembled by a joint 3 to form the main beam 1 of semi-trailer for transporting 53′ container.
The conventional joint is a connecting plate welded on a cross section of a main beam of the semi-trailer. An I-shaped main beam of container semi-trailer in FIG. 2 shown as example illustrates the structural of the conventional joint. The joints are two connecting plates 6 respectively welded on the cross-section of the front beam 2a and the rear beam 2b of the main beam. Each of the connecting plates 6 has six bolt holes 8, therefore bolts can be used to connect the front beam 2a and the rear beam 2b by the connecting plates 6. Additionally, two ribs 7 are disposed on the inner side of connecting plate 6 which is connected to the web plate of the I-shaped main beam to prevent the connecting plates from deforming after they are connected by bolts. FIG. 3 illustrates the main beam of container semi-trailer provided with the conventional joints. The connecting of the main beam of container semi-trailer follows the following steps. First two joints 6 of the front beam 2a and rear beam 2b of the main beam are aligned with each other, and six bolts are inserted into six bolt holes 8 respectively and fastened by a fastening tool to form a complete main beam. This is a simple and practical connecting method. However, it can be clearly seen from the figures that with this structure, the bolts bear all the forces transferred to the joint when the semi-trailer is running. During running of the semi-trailer, the main beam is often in situations of being stretched, bent, twisted, or sheared etc. All of these operating conditions are directly transferred to the bolts, resulting in that strength of the main beam is actually assured by the strength of the bolts. However, in various operating conditions, the forces on the bolts are asymmetric. Therefore, some bolts will bear a very large force which may exceed the yield limit of the bolts so that the bolts are deformed or destroyed. Additionally, when the main beam is bent, the tension of the bolts adjacent to the pulling side of the joint is increased because the pretightening force is superimposed by a new pulling force. The pretightening force of bolts adjacent to the pressing side is decreased, and this may loosen the bolts. In practice, it may be an alternate process, i.e. a certain bolt is tightened and loosened alternately which may result in a failed connection.
As mentioned above, it is necessary to design a reliable joint suitable for container transportation of a segmented semi-trailer.